A wide-area positioning system, such as the global positioning system (GPS), uses a constellation of satellites to position or navigate objects on earth. Currently, the constellation of satellites broadcast signals at two carrier frequencies, L1 frequency at (154*10.23 MHz) or 1572.45 MHz and L2 frequency at (120*10.23 MHz) or 1227.6 MHz, or L1 wavelength of 0.1903 m and L2 wavelength of 0.2442 m, respectively. For each carrier frequency, two types of GPS measurements are usually made by a GPS receiver with an object to be positioned. The two types of GPS measurements are pseudorange measurement, and integrated carrier phase measurement. The pseudorange measurement (or code measurement) is a basic GPS observable that all types of GPS receivers can make. It utilizes the C/A or P codes modulated onto the carrier signals. The carrier phase measurement is obtained by integrating a reconstructed carrier of the signal as it arrives at the receiver. Because of an unknown number of whole cycles in transit between the satellite and the receiver when the receiver starts tracking the carrier phase of the signal, there is a whole-cycle ambiguity in the carrier phase measurement. This whole-cycle ambiguity must be resolved in order to achieve high accuracy in the carrier-phase measurement.
With the GPS measurements available, the range or distance between a GPS receiver and each of a plurality of satellites is calculated by multiplying a signal's travel time by the speed of light. These ranges are usually referred to as pseudoranges (false ranges) because the GPS measurements may include errors due to various error factors, such as satellite clock timing error, ephemeris error, ionospheric and tropospheric refraction effects, receiver tracking noise and multipath error, etc. To eliminate or reduce these errors, differential operations are typically used in GPS applications. Differential GPS (DGPS) operations typically involve a base reference GPS receiver, a user GPS receiver, and a communication mechanism between the user and reference receivers. The reference receiver is placed at a known location and the known position is used to generate corrections associated with some or all of the above error factors. The corrections generated or raw date measured at the reference station are supplied to the user receiver, which then uses the corrections or raw data to appropriately correct its computed position. Differential operations using carrier-phase measurements are often referred to as real-time kinematic (RTK) positioning/navigation operations.
The corrections generated or raw data measured at the reference receiver, however, are useful at the user GPS receiver only when there are spatial and temporal correlations of the errors between reference receiver and the user receiver. While the GPS satellite clock timing error, which appears as a bias on the pseudorange or carrier phase measurement, is perfectly correlated between the reference receiver and the user receiver, most of the other error factors are either not correlated or the correlation diminishes in wide-area applications, i.e., when the distance between the reference and user receivers becomes large. Moreover, when the distance between the user receiver and the reference receiver becomes large, such as more than about 10 to 20 kilometers, the two carrier frequencies in the existing GPS system are inadequate to resolve the whole-cycle carrier-phase ambiguities.